1. Field of the Invention
The present invention relates to a digital image forming apparatus with use of electrophotographic process such as a digital printer or a digital copying machine.
2. Description of the Prior Art
In an image forming apparatus such as a laser printer, a laser diode is driven to expose a photoconductor according to digital image data converted from analog image signals and an image is reproduced with use of an electrophotographic process. Various digital image forming methods of exposing the photoconductor to reproduce a halftone image such as a photograph with a high fidelity are proposed for such an image forming apparatus.
Digital image forming methods includes an area gradation method with use of a dither matrix, a multi-value laser exposure method such as a pulse width modulation method or an intensity modulation method, and a multi-value dither method which combines the dither method with a pulse width modulation method or an intensity modulation method (refer for example Japanese Patent laid open Publications 62-91077/1987, 62-39972/1987, 62-188562/1987 and 61-22597/1987).
By using such a gradation method, the image density can be reproduced in principle in correspondence to the gradation of an image to be reproduced. However, the reproduced image density with a gradation method is not proportional correctly to the original image density due to the photosensitive characteristics of a photoconductor and the characteristics of toners. Then, the reproduced image density does not change linearly with the original image density and the resultant nonlinear characteristic is generally called as the gamma characteristic. The nonlinearity is a large factor which lowers the fidelity of the reproduced image of a half-tone document.
Then, the so-called gamma correction is conducted to improve the nonlinearity. That is, the document density read with a sensor is converted with a conversion table (called as gamma table) for the gamma correction and a digital image is formed according to the converted document density in order to realize the linear characteristics. By using such a gamma table, the image can be reproduced according to the document density level with a fidelity.
Further, there are other factors which affect the image density. The amount of adhered toners on the photoconductor on development changes according to the external environment of the photoconductor such as temperature or humidity owing to the characteristics of the photoconductor and the toners. That is, the transfer efficiency varies with the environment conditions. For example, in an environment of high temperature and high humidity, the adhered amount of toners increases, and the slope of the gamma characteristic at low and intermediate image densities is known to increase so as to make the reproduced image darker. On the other hand, in an environment of low temperature and low humidity, the adhered amount of toners decreases, and the slope of the gamma characteristic at low and intermediate image densities is known to decrease so as to make the reproduced image lighter.
As mentioned above, the density of a reproduced image varies also according to the environment. In order to solve this problem and to stabilize the reproduced image, the density is generally controlled so as to make the maximum density constant. In a conventional automatic density control, a standard toner image for the density control is first formed on the surface of the photoconductor, and the toner density of the standard toner image is measured by detecting the quantity of reflection light with a sensor arranged near the photoconductor. The grid potential of the sensitizing charger, the development bias potential of the development unit and/or the maximum light intensity of laser beam are changed according to the detected value by the sensor. Previously, the density control is performed by keeping the difference between the surface potential V.sub.0 and the development bias potential V.sub.B constant in order to prevent a fog in the background of an image and to prevent the adhesion of carriers of two-component development material to the photoconductor. However, though such density control can make the maximum density constant, it is a problem that the gamma characteristic changes largely when the surface potential V.sub.0 and the development bias potential V.sub.B are changed by keeping the difference between the surface potential V.sub.0 and the development bias potential V.sub.B constant. In other words, the reproduced image does not have the same gradation characteristic.
Then, it is proposed to compare a grey scale obtained by copying a standard document with the standard grey scale itself for comparison to detect the optical densities of the two electrostatic latent images formed on a photoconductor. The correction amount of the toner density is calculated from the comparison of the two data, and the result is shown in a display so that a user can correct the dial setting for toner density (refer for example JP-A 141645/1979). Thus, the reproduced image can be kept to have the initial quality. However, the gradation is controlled indirectly in this method. Then, it is not guaranteed that a desired image can be obtained after the control. Further, the control cannot be performed in real time according to the changes of the copying process.